Hitherto, R-T-B system rear earth permanent magnets (hereinafter, may be referred to as “R-T-B system magnet”) have been used in motors such as voice coil motors of hard disk drives. Motors having an R-T-B system magnet incorporated in a rotor exhibit high energy efficiency. In recent years, demands for energy conservation have increased and the heat resistance property of R-T-B system magnets has been improved, whereby the amount of R-T-B system magnets which are used in various motors of home electronics, air conditioners, vehicles, and the like increases.
Generally, R-T-B system magnets are obtained by molding and sintering an R-T-B system alloy containing Nd, Fe, and B as main components. Generally, in R-T-B system alloys, R is Nd, a part of which is replaced by other rare earth elements such as Pr, Dy, and Tb. T is Fe, a part of which is replaced by other transition metals such as Co and Ni. B is boron and a part thereof can be replaced by C or N.
Generally, R-T-B system magnets are constituted by two phases, i.e., a main phase constituted by R2T14B and an Nd-rich phase which is present at the grain boundaries of the main phase and has a higher Nd concentration than the main phase. The Nd-rich phase is also referred to as a grain boundary phase.
Since R-T-B system magnets which are used in motors of hybrid vehicles, electric vehicles, and the like are exposed to high temperatures in the motors, high coercivity (Hcj) is required. There is a technique for replacing R of an R-T-B system alloy from Nd to Dy as a technique for improving the coercivity of the R-T-B system magnet. However, Dy is unevenly distributed and its output is also limited. Accordingly, the supply of Dy is unstable.
Therefore, techniques for improving coercivity of an R-T-B system magnet without increasing the content of Dy contained in an R-T-B system alloy are examined. As such techniques, a method for depositing Dy from the outside of a sintered body of an R-T-B system alloy and spreading Dy to interior grain boundaries (for example, see PTLs 1 and 2), a method for applying fluoride of Dy or the like to a surface of a sintered body of an R-T-B system alloy (for example, see PTL 3), a method for obtaining a core shell-type structure by adding a raw material having a high Dy concentration (for example, see NPL 1), and the like are examined.
In addition, regarding an expression mechanism of coercivity of an R-T-B system magnet, results of analysis on effects due to a heat treatment are reported. Specifically, there is a report in which a very thin amorphous layer is formed at the grain boundaries due to a heat treatment and a Cu-condensed phase is thus present together with an Nd-rich phase (for example, see NPL 2), or in which wettability between Nd-rich phases is improved due to a liquid phase which is present due to the presence of Cu (for example, see NPL 3).
In addition, generally, when producing an R-T-B system magnet, an alloy having a microstructure is pulverized into grains having a size of 4 μm to 6 μm using a jet mill or the like, and molded and sintered while being oriented in a magnetic field.
In addition, an effort is also made to make the powder into a finer powder having a grain size of up to 3 μm or less in the pulverization of the alloy to thereby reduce the size of the magnet grains obtained after sintering, thereby improving coercivity and thus reducing the amount of Dy (for example, see PTL 4).
In addition, a method for producing a rare earth sintered magnet including the steps of: preparing a mixed powder by mixing a first fine alloy powder and a second fine alloy powder having different compositions; obtaining a green body by pressing the mixed powder while applying a reversal magnetic field as an orienting magnetic field; and obtaining a sintered body by sintering the green body is proposed as a technique for obtaining a sintered magnet having a high orientation rate (for example, see PTL 5).